Generally, for a distributed storage system, for example, Redundant Array of Independent Disks (RAID) devices are typical. In a RAID device, data is stored in a plurality of storages in a distributed manner.
Japanese Laid-open Patent Publication No. H.08-030495 discusses that when a directory management device is informed by a management terminal that a number of usable storage devices has increased, the directory management device finds segments to be moved, informs a data management device of arrangement information of the segments before and after movement, and requests the movement of the segments. The data management device which receives the request stores the arrangement information of the segments required for the movement in a movement information storage unit, and copies the segments from the movement-source storage device to a movement-destination storage device when access to the storage devices decreases. After the segments are copied, completion of the movement is reported to the directory management device.
Japanese Laid-open Patent Publication No. 2008-204206 discusses that a split data management unit determines and manages arrangement destinations of a plurality of duplicate data corresponding to respective split data configuring a content. The split data management unit performs the deletion, addition, or rearrangement of the split data in a predetermined timing. When an access to the content from a host is requested, the split data management unit determines the access destinations of the split data configuring the content. A number of duplication planning unit determines the number of duplication of the respective split data and defines all available storage devices as arrangement destination candidates, and determines arrangement so that the plurality of duplicate data corresponding to the same split data are arranged equally to the respective storage devices.
Japanese Laid-open Patent Publication No. 2005-209055 discusses that a storage having a plurality of groups of volumes with a plurality of logic volumes to which input/output are performed exclusively has a load monitoring unit that monitors an imbalance of the loads of the logic volumes. When the load monitoring unit detects an imbalance of the loads, a certain logic volume is moved to another logic volume within another group of logic volumes in which the content of the logic volumes within the group of volumes of the greater load is copied into a preliminary logic volume in the group of volumes with the smaller load.
In typical RAID devices, a bandwidth of a data transmission line is substantially constant regardless of time of day and data input/output performance of a plurality of storages is substantially the same. Under the presuppositions, the RAID device achieves a faster access by storing data in each of the storages substantially uniformly compared with when data is input/output to and from a single storage.
However, with improvement of network performance, a plurality of storages and a processing device such as a server are not connected by a dedicated transmission line but connected by networks such as Local Area Network (LAN) and Storage Area Network (SAN) in many cases. In a distributed storage system under the network environment, bandwidths of networks which are data transfer routes may be dynamically changed.